Quantitative studies of brain morphology in groups of normal children aged eight to 10 years and of young adults aged 25 to 39 years revealed continuing maturation of the brain over this age-range. There was some evidence of slightly increasing brain volume, but the most striking changes occurred in the gray matter to white matter ratio, and especially in the cortical mantle, which becomes substantially thinner on MRI between the age of eight and adulthood. An increase in the volume of the cerebral ventricles occurred in close association with the decrease in cortical gray matter. Analyses of the morphology of the cerebellar vermis suggest that significant increases occur in the neocerebellar vermal area over this age-range, and that the ratio of neocerebellar to paleocerebellar vermal areas may also increase.